In the past, it has proved difficult to blend a thick film resistor paint having sheet resistivity in the megohm per square range, that also exhibited TCR in the range of .+-.200 ppm/.degree.C., and thermal stability of less than 0.25%. Palladium or plladium oxide (with or without silver) has been used to make resistors yielding good resistance properties only in the 10K to 100K ohms/square range.
Three alternate approaches have been used with ruthenium oxide resistors to raise the resistance properties of the resistor paint to the megohm range. One approach was to add more glass frit. A second approach was to mix inert materials with a higher melting point than the glass frit used. The third approach was to add sufficient semiconductive materials to the resistor paint to raise the resistance properties into the megohm range.
These three approaches were only partially successful; resulting in interface problems, increased surface roughness, poor thermal stability, high short time overload, and excessively negative TCR.